The present invention relates to a liquid crystal display device wherein two liquid crystal panels, each of which includes a liquid crystal developing a memory effect, are provided in layers. More specifically, the present invention relates to a liquid crystal display device providing black and white display and to a liquid crystal display device capable of providing clear red display without degrading the brightness of black and white display.
It has been known that three liquid crystal panels having different selective reflection wavelengths of R, G and B can be provided in layers in order to provide black and white display by liquid crystal panels utilizing chiral nematic crystals or cholesteric liquid crystals (liquid crystals developing a memory effect). However, it is practically preferable in terms of driving voltage, cost or another factor that the liquid crystal panels are provided in two layers.
A chiral nematic crystal, which is sandwiched between a pair of transparent electrode substrates, has a structure that its director turns through one revolution while twisting at a certain cycle (hereinbelow, referred as to the helical pitch). When the central axes in the twisted structure (hereinbelow, referred as to the helical axes) are averagely aligned perpendicular to the substrates, circular polarized light is reflected corresponding to the twist direction.
The central wavelength of the reflected light is almost equal to the product of the helical pitch and the average refractive index of the liquid crystal composition. The helical pitch is determined according to the formula of p=1/(cxc2x7HTP) based on the adding amount c of a photoactive material, such as a chiral agent, and the constant of the photoactive material HTP (Helical Twisting Power). Thus, the reflection wavelength can be arbitrarily controlled by selection of the type of the photoactive material and the adding amount.
The phenomenon that a chiral nematic crystal reflects circular polarized light having a specific wavelength that depends on the helical pitch and the refractive index of the liquid crystal is called selective reflection. The liquid crystal alignment that exhibits the selective reflection is called a planar alignment, wherein the average direction of the helical axes in a plurality of separated liquid crystal domains is substantially perpendicular to the substrate surfaces.
A chiral nematic crystal can have another liquid crystal alignment (focal conic structure) different from the alignment just stated, wherein the helical axes in a plurality of liquid crystal domains face to random directions or a non-perpendicular direction to the substrate surfaces. The liquid crystal layer in a focal conic state is in a slightly scattered state as a whole and does not reflect light having a specific wavelength as in the selective reflection.
Even when no electric field is applied, both states (the planar state and the focal conic state) are stable. The selective reflection in the planar state is bright due to no need for a polarizer and provides a wide viewing angle. A liquid crystal display element that includes a chiral nematic crystal to utilize the selective reflection thereof can be provided as a liquid crystal display element having lower power consumption since the liquid crystal alignment can be maintained to make the element function as a type developing a memory effect even when no electric field is applied.
The applied voltage can be controlled to exhibit a state having an intermediate optical property between the planar alignment and the focal conic alignment after voltage application or to have, in the transitional process, a homeotropic state wherein crystal molecules are aligned in the direction of an electric field.
When liquid crystal display elements having a chiral nematic liquid crystal are used, full color display can be provided by fabricating three types of liquid crystal panels so as to have different liquid crystal pitches and exhibit different sorts of color selective reflection of R, G and B, and arranging these panels one after another. When the panels are put one after another, the panels are normally put one after another in order of shortness in the selective reflection wavelength, such as the order of blue, green and red, as viewed from the display side in terms of display quality (see, e.g., JP-A-11-64895).
Two types of liquid crystal panels can be combined to provide multicolor display, though the combination cannot provide full color display. In the case of combining two types of liquid crystal panels, a specific combination of selectively reflected colors can provide black and white display.
However, the provision of multicolor display by a combination of two layers of liquid crystal panels has created several problems from turbidity in a displayed color or a degrade in color purity since black and white display varies according to, in particular, the panel structure, the liquid crystal materials or the like. Especially, it is one of the key issues to display white in achromatic fashion. It is a first object of the present invention to resolve this issue.
By the way, it is difficult to realize both clear black and white display having high contrast and red display having high color purity by only combining a liquid crystal panel having a central wavelength (dominant wavelength) in selective reflection on a long wavelength side and a liquid crystal panel having a central wavelength (dominant wavelength) in selective reflection on a short wavelength side in a two-layered structure.
This is because when the selective reflection wavelength is set on a long wavelength side to obtain red display, the helical axes of the liquid crystals face to variety of directions, light is widely scattered in a planar state, the half width of the selective reflection wavelength is widened to provide a broad range of reflection property, and the color purity is lowered to make a shift from yellow to brownish color.
Even in the case of a two-layered structure, an auxiliary color filer can be provided between a liquid crystal panel on the long wavelength side and a liquid crystal panel on the short wavelength side to realize both clear black and white display and red display. In this case, it is an important issue to prevent the brightness and the contrast in black and white display from degrading.
From this viewpoint, it is a second object of the present invention to provide a liquid crystal display device capable of realizing multi-color display including good black and white display and clear red display when two liquid crystal panels having liquid crystals developing a memory effect are provided in layers so as to sandwich a color layer therebetween in order to black, white and red display.
In order to solve the objects, according to a first aspect of the present invention, there is provided a liquid crystal display device comprising a first liquid crystal panel and a second liquid crystal panel; the first liquid crystal panel and the second liquid crystal panel exhibiting at least two stable states of a planar state and a focal conic state with no voltage applied; the first liquid crystal panel and the second liquid crystal panel including liquid crystals developing a memory effect, the liquid crystals having selective reflection wavelengths in the planar state appearing in a visible range, the first liquid crystal panel and the second liquid crystal panel having different selective reflection wavelengths; the first liquid crystal panel having a shorter selective reflection wavelength and the second liquid crystal panel having a longer selective reflection wavelength being combined to be respectively provided on a viewer side and on a no-viewer side; the selective reflection wavelength of the first liquid crystal panel being set in a range from 430 nm to 540 nm, and the selective reflection wavelength of the second liquid crystal panel being set in a range from 560 nm to 665 nm, whereby at least black display and white display can be provided.
It is preferable that chromaticities lie in a region surrounded by four points of (0.29, 0.29), (0.35, 0.29), (0.29, 0.36) and (0.35, 0.36) on C.I.E. 1931 chromaticity coordinates, the chromaticities being observed by a viewer when the first and second liquid crystal panels are provided in layers both panels being in the planar state.
It is preferable that the selective reflection wavelength of the first liquid panel is in a range from 430 nm to 510 nm, and the selective reflection wavelength of the second liquid panel is in a range of from 560 nm to 660 nm.
It is preferable that the liquid crystal of the first liquid panel and the liquid crystal of the second liquid panel have a color layer having a certain transmissive property, whereby display in a third color can be provided in addition to the black display and the white display.
It is preferable that the selective reflection wavelength of the first liquid panel is in a range from 490 nm to 540 nm, the selective reflection wavelength of the second liquid panel is in a range of from 615 nm to 665 nm, and a color layer in red is provided for red display.
It is preferable that a transmission property of the color layer is set such that a transmissivity at a wavelength shorter than the selective reflection wavelength of the second liquid crystal panel by 70 nm is not higher than 85% of a transmissivity at a wavelength shorter than the selective reflection wavelength by 30 nm.
It is preferable that red display is provided by a color lying in a region delimited by connecting four points of (0.47, 0.30), (0.60, 0.30), (0.47, 0.40) and (0.60, 0.40) on C.I.E. chromaticity coordinates, and that the white display is provided by a color lying in a region delimited by connecting four points of (0.29, 0.29), (0.35, 0.29), (0.29, 0.44) and (0.35, 0.44) on the C.I.E. chromaticity coordinates.
According to a second aspect of the present invention, there is provided a liquid crystal display device comprising a first liquid crystal panel and a second liquid crystal panel; the first liquid crystal panel and the second liquid crystal panel exhibiting at least two stable states of a planar state and a focal conic state with no voltage applied; the first liquid crystal panel and the second liquid crystal panel including liquid crystals developing a memory effect, the liquid crystals having selective reflection wavelengths in the planar state appearing in a visible range, the first liquid crystal panel and the second liquid crystal panel having different selective reflection wavelengths; the first liquid crystal panel having a shorter selective reflection wavelength and the second liquid crystal panel having a longer selective reflection wavelength being combined to be respectively provided on a viewer side and on a no-viewer side; and a color layer provided between the liquid crystal of the first liquid crystal panel and the liquid crystal of the second liquid crystal panel, the color layer having a transmission property that is set such that a transmissivity at a wavelength shorter than the selective reflection wavelength of the second liquid crystal panel by 70 nm is not higher than 85% of a transmissivity at a wavelength shorter than the selective reflection wavelength by 30 nm.
It is preferable that when the selective reflection wavelength of the second liquid panel is in a range from 630 nm to 640 nm, the color layer provided between the second liquid crystal panel and the first liquid crystal panel has a transmission property that is set such that a transmissivity at a wavelength in a range from 560 nm to 570 nm is not higher than 85% of a transmissivity at a wavelength in a range from 600 nm to 610 nm.
It is preferable that when the selective reflection wavelength of the second liquid panel is in a range from 645 nm to 655 nm, the color layer provided between the second liquid crystal panel and the first liquid crystal panel has a transmission property that is set such that a transmissivity at a wavelength in a range from 575 nm to 585 nm is not higher than 70% of a transmissivity at a wavelength in a range from 615 nm to 625 nm.
It is preferable that the liquid crystals developing a memory effect are chiral nematic liquid crystals or cholesteric liquid crystals.
According to a third aspect of the present invention, there is provided a method for preparing a liquid crystal display device, which includes liquid crystals developing a memory effect, the liquid crystals exhibiting at least two stable states of a selective reflection state and a transmissive state with no voltage applied; and a first liquid crystal panel and a second liquid crystal panel provided in layers, the first liquid crystal panel and the second liquid crystal panel having different selective reflection wavelengths; the method comprising providing a color layer on a rear side; controlling the selective reflection wavelengths of the respective liquid crystal panels so that a point at a right middle between coordinates corresponding to two developed colors lies at a position apart from a white point under a standard light source by a distance of not greater than 0.03 on C.I.E. xy chromaticity coordinates when the two developed colors are indicated on the xy chromaticity coordinates with one of the liquid crystal panels being in the selective reflection state, and the other being in the transmissive state; and providing white by mixing at least the two developed colors and a color (except white) developed by the color layer.
It is preferable that the method further comprises providing a filter layer having a specific transmissive color between the first liquid crystal panel and the second liquid crystal panel; and providing at least the color (except white) developed by the color layer, white prepared by the two developed colors and the filter layer, and red prepared by the filter layer.
It is preferable that the method further comprises providing the filter layer by use of a resist having a light transmissive property; providing the filter layer in contact with the liquid crystal of the liquid crystal panel on a no-viewer side; and providing the filter layer under a driving electrode.
It is preferable that red display lies in a range of X=0.43 to 0.50 and y=0.29 to 0.37 on the xy chromaticity coordinates.
It is preferable that a pigment resist is provided as the color layer.
It is preferable that the standard light source is C light source.